The present invention relates to the manufacture of nickel hypophosphite for use in electroless nickel plating, and more particularly, to the manufacture of nickel hypophosphite by anodically dissolving nickel metal into a solution containing hypophosphite anions. The present invention provides a significant improvement in the means by which nickel hypophosphite may be produced efficiently and inexpensively.
Nickel phosphorus alloys have been deposited for many years by chemical reduction from solutions known in the industry as "electroless nickel plating baths." The deposits from these baths are in widespread use throughout industry on parts as large as oil drilling equipment and as small as watch movements.
The conventional electroless nickel plating baths used throughout the industry contain nickel sulfate, sodium hypophosphite, chelating or complexing agents, buffers and stabilizers. Replenishment is made with the same materials, as the bath is used. These plating solutions have limited life because there is a continual build-up of sulfate ion, sodium ion and phosphite ions from the oxidation of hypophosphite ion.
Attempts have been made to formulate electroless nickel plating baths with nickel hypophosphite and hypophosphorous acid, thus eliminating both sodium and sulfate ions, thereby giving the bath longer life. Baths of this type have been available in prior art literature and at least two commercial electroless nickel processes have been proposed using this type of formulation. The problem with these formulations is that nickel hypophosphite is very expensive and the increased cost of these processes does not warrant their commercial use, even though they represent improvements over the conventional formulations.
Nickel hypophosphite can be made by reacting hypophosphorous acid with nickel hydroxide or nickel carbonate or some other suitable nickel salt. The problem with this method is the high cost and limited availability of hypophosphorous acid. Furthermore, nickel in the form of the above salts is considerably more expensive than nickel in the form of sulfate that is in commercial use.
Czech Patents 139,245 and 152,901 describe methods for producing nickel hypophosphite which do not use the expensive hypophosphorous acid. In these patents, nickel nitrate in aqueous solution is mixed with aqueous sodium hypophosphite. Nickel hypophosphite crystallizes out of solution because it is the least soluble material in the mixture and is easily separated. This method of producing nickel hypophosphite is very simple; however, there is enough sodium hypophosphite and nickel nitrate left in solution to cause this method to be very expensive.
There remains a need for an efficient, inexpensive means by which nickel hypophosphite may be manufactured.